This invention relates to the use of solid electrolyte ionic conductor systems, and in particular to oxygen-selective ion transport membranes (OTM""s) in gas separation systems and most particularly to the use of OTM""s in gas purification systems.
Non-cryogenic bulk oxygen separation systems, for example, organic polymer membrane systems, have been used to separate selected gases from air and other gas mixtures. Air is a mixture of gases which may contain varying amounts of water vapor and, at sea level, has the following approximate composition by volume: oxygen (20.9%), nitrogen (78%), argon (0.94%), with the balance consisting of other trace gases. An entirely different type of membrane, however, can be made from certain inorganic oxides. These solid electrolyte membranes are made from inorganic oxides typified by calcium- or yttrium-stabilized zirconium and analogous oxides having a fluorite or perovskite structure.
Although the potential for these oxide ceramic materials as gas separation membranes is great, there are certain problems in their use. The most obvious difficulty is that all of the known oxide ceramic materials exhibit appreciable oxygen ion conductivity only at elevated temperatures. They usually must be operated well above 500xc2x0 C., generally in the 900xc2x0 C. to 1100xc2x0 C. range. This limitation remains despite much research to find materials that work well at lower temperatures. Solid electrolyte ionic conductor technology is described in more detail in Prasad et al., U.S. Pat. No. 5,547,494, entitled Staged Electrolyte Membrane, which is incorporated by reference in its entirety herein to more fully describe the state of the art. The elevated temperatures of operation, however, make ion transport processes intrinsically well suited for integration with high temperature processes such as vapor-based, gas-based, or combined power cycles.
Recent developments have produced solid oxides which have the ability to conduct oxygen ions at elevated temperatures if a chemical driving potential is applied. The chemical driving potential is established by maintaining an oxygen partial pressure difference across the material. These pressure-driven ionic conductor materials may be used as membranes for the extraction of oxygen from oxygen-containing gas streams if a sufficiently high ratio of oxygen partial pressures is applied to provide the chemical driving potential. Namely, the oxygen partial pressure is maintained at a higher value on the cathode side of the membrane, that is exposed to the oxygen-containing gas, than on the anode-side, where oxygen transported through the material is recovered.
The membranes have xe2x80x9coxygen selectivityxe2x80x9d. Oxygen selectivity is the tendency of the membrane to transport oxygen ions in preference to other elements and ions thereof. Since the selectivity of these materials for oxygen is infinite (a total preference for transporting oxygen ions to the exclusion of other ions), and oxygen fluxes several orders of magnitude higher than that for polymeric membranes can be obtained, attractive opportunities are created for the production of oxygen as well as for oxygen-requiring oxidation processes, especially with applications that involve elevated temperatures. A prominent example is gas turbine cycles which typically process a significant amount of excess air to keep the turbine inlet temperature within the capabilities of available materials and therefore make available excess oxygen for recovery as a co-product.
Some of the key problems that have to be addressed in the design of ion transport membrane systems and their integration into a high temperature cycle such as a gas turbine involve maximizing driving forces for ion transport, minimizing gaseous diffusion resistance, avoiding excessive stresses from thermal and compositional expansion and contraction and sealing the ion transport elements within the ion transport apparatus. The latter problem is aggravated by ion transport membrane operating temperatures being in the range from 800xc2x0 C. to 1100xc2x0 C.
Advances in the state of the art of air separation using solid electrolyte ionic conductors have been presented in the technical literature. For example, Mazanec et al., U.S. Pat. No. 5,306,411, entitled Solid Multi-Component Membranes, Electrochemical Reactor Components, Electrochemical Reactors and Use of Membranes, Reactor Components, and Reactor for Oxidation Reactions, relates to electrochemical reactors for reacting an oxygen-containing gas with an oxygen-consuming gas and describes a shell and tube reactor with the oxygen-consuming gas flowing on one side of the solid electrolytic membrane and the oxygen consuming gas on the other. Mazanec et al., however, does not address issues related to integrating such systems with oxygen production from gas turbine cycles, heat management to maintain membrane surfaces at the desired uniform temperatures, flow dynamics to achieve effective mass transfer, or the need for balancing reaction kinetics with oxygen ion conductivity to maintain the appropriate oxygen partial pressure for materials stability.
Gottzmann et al., U.S. Pat. No. 5,820,655, entitled Solid Electrolyte Ionic Conductor Reactor Design, describes an ion transport reactor and process using an ion transport membrane for extracting oxygen from a feed gas stream flowing along its retentate side. A reactant gas flows along the permeate side to react with the oxygen permeated through the membrane. Provisions are included to transfer the heat of the anode side reaction to a fluid stream flowing through the reactor in a fashion which maintains the membrane operating temperature within its operating range. The patent is silent on the recovery of carbon dioxide from the reacted permeate stream.
Prasad et al., U.S. Pat. No. 5,837,125, entitled Reactive Purge for Solid Electrolyte Membrane Gas Separation, describes a system and process for obtaining high purity oxygen-free products from an oxygen containing feed stream by permeating the contained oxygen to the permeate side of an oxygen ion transport membrane where the permeated oxygen reacts with a reactant purge stream to establish a low partial oxygen pressure at the anode. This permits removal of oxygen down to very low concentrations. The patent is silent on production and recovery of carbon dioxide.
Kang et al., U.S. Pat. No. 5,565,017, entitled High Temperature Oxygen Production with Steam and Power Generation, relates to a system integrating an ion transport membrane with a gas turbine to recover energy from the retentate gas stream after it is heated and steam is added. The retentate gas stream is the stream on the cathode side of the membrane following contact with the membrane wherein a portion of the elemental oxygen is transported through the membrane, while a permeate gas stream is on the anode side and receives such transported oxygen. Oxygen transported across the membrane from the cathode side to the anode side is designated as permeate oxygen or a permeate oxygen portion of the oxygen initially contained on the cathode side. The injection of steam or water into the ion transport retentate stream compensates for the loss of the oxygen mass from the turbine feed gas stream.
Kang et al., U.S. Pat. No. 5,562,754, entitled Production of Oxygen By Ion Transport Membranes with Steam Utilization, discloses a system integrating an ion transport membrane with a gas turbine to recover energy from the retentate gas stream after it is heated. Steam is added as a sweep gas on the anode side to enhance oxygen recovery. A stream containing a mixture of oxygen and steam is produced on the anode side which can be withdrawn as a product.
Kang et al., U.S. Pat. No. 5,516,359, entitled Integrated High Temperature Method for Oxygen Production, describes heating a compressed air feed gas stream to the appropriate ion transport operating temperature by a first combustor which, in one embodiment, is inserted between the compressor discharge and the ion transport separator. Subsequently, the retentate gas stream from the ion transport separator is heated to turbine inlet temperature by a second combustor. The inlet temperatures to the turbine and to the ion transport separator are independently controlled by adjusting the fuel rates to the two combustors. In an alternate configuration, a heat exchanger is placed between the two combustors, which are both located downstream from the ion transport separator, and the air feed gas stream to the ion transport separator is heated to the correct temperature by this heat exchanger.
The method disclosed in the Kang et al. ""359 patent has the disadvantage that the feed gas stream to the ion transport separator contains products of combustion which dilute the ion transport separator feed gas stream, reduce the oxygen driving force, and could act as an adverse contaminant to some mixed conductor materials. Because some of the oxygen contained in the feed air is consumed in the first combustor, the oxygen driving force is further reduced. The alternative configuration has the disadvantage of requiring an additional, potentially expensive, heat exchanger. Either method could also have a problem maintaining stable combustion in the first combustor since the fuel-to-air equivalence ratio will be low, especially where the heat contained in the turbine exhaust is regeneratively recovered by the feed air gas stream and the heat duty of the first combustor is small.
Hegarty, U.S. Pat. No. 4,545,787, entitled Process for Producing By-Product Oxygen from Turbine Power Generation, relates to a process for generating net power using a combustion turbine, accompanied by the recovery of by-product oxygen-enriched gas. Air is compressed and heated, at least a portion of the air is combusted, and a portion of the oxygen is removed from the air or combustion effluent using an air separator. The oxygen lean combustion effluent is expanded through a turbine to produce power. In an alternative embodiment, the effluent from the turbine is used to produce steam to generate additional power. In this process, the type of fuel is generally limited to xe2x80x9ccleanxe2x80x9d fuels such as natural gas, oils, or synthesis gas. The term synthesis gas (xe2x80x9csyngasxe2x80x9d) means a mixture consisting essentially of hydrogen and carbon monoxide and often additional impurities with a molar ratio of hydrogen to carbon monoxide of from about 0.6 to about 6. Syngas is a common intermediate in the conversion of natural gas into liquid fuels such as methanol, formaldehyde and olefins.
Chen, U.S. Pat. No. 5,035,727, entitled Oxygen Extraction from Externally Fired Gas Turbines, relates to a process for recovering high purity oxygen from an externally fired power generating gas turbine cycle.
While this process is similar to the Hegarty ""787 patent, Chen differs in the use of an externally fired gas turbine so that other types of fuel such as coal or biomass may be used.
It is therefore an object of the invention to provide a process for production of carbon dioxide. It is a further object of the invention to provide the carbon dioxide in a relatively pure form.
It is a further object of the invention to provide effective heat management of the reactors utilized to produce the carbon dioxide.
It is a further object of the invention to provide such a process which is economically efficient to manufacture and operate. It is a further object of the invention to achieve such efficiency via appropriate cogeneration of energy and/or additional useful products such as nitrogen and oxygen.
In a first aspect, the apparatus is directed to a method for the production of carbon dioxide. An oxygen-containing first process gas is flowed along a cathode side of a first oxygen selective ion transport membrane. The membrane is at operating conditions effective to transport a first permeate oxygen portion from the cathode side to an opposite anode side. A carbon-containing second process gas is flowed along the anode side at a flow rate effective to provide fuel lean conditions that is at a stoichiometric surplus of oxygen for reacting with the first permeate oxygen portion. A first mixture of the second process gas and the first permeate oxygen portion is combusted such that substantially all of the second process gas is converted into a second mixture of water and carbon dioxide. The carbon dioxide is separated from such second mixture.
In preferred embodiments of this first aspect, the carbon-containing second process gas may be selected from the group consisting of hydrocarbons, carbon monoxide, alcohols and mixtures thereof. Such second process gas may be a paraffinic hydrocarbon. A purge gas may be combined with the second process gas prior to the combusting. The purge gas may be steam. The combustion may be conducted downstream of the first anode side within the permeate passage of the reactor or downstream from the reactor.
A portion of a first retentate portion of the first process gas may be flowed along a cathode side of a second membrane, having opposite second cathode and anode sides, that is at operating conditions effective to transport a second permeate oxygen portion from the second cathode side to the second anode side. A product gas may be recovered from a second retentate portion. The product gas may be selected from the group consisting of nitrogen, argon, and mixtures thereof. A second reactive purge gas may be flowed along the second anode side. The second purge gas may contain a low grade fuel gas and diluent gases selected from the group consisting of steam and a remainder of the second retentate portion. The second reactive purge gas may be reacted with the second permeate oxygen portion in a complete or partial oxidation reaction. A product of the partial oxidation reaction may be flowed along the first anode side. The second reactive purge gas may be natural gas and syngas may be recovered as a product of the partial oxidation reaction. One of the advantages of the arrangement is that the second stage may be operated under fuel rich conditions to assure more complete removal of oxygen from the second retentate stream.
In a second aspect, the invention is directed to a process for the cogeneration of carbon dioxide, nitrogen and electrical power. A pressurized oxygen-containing first process gas is flowed along a cathode side of a first oxygen selective ion transport membrane that is operating at conditions effective to transport a first permeate oxygen portion from the cathode side to an opposing anode side. A purge gas optionally containing fuel is introduced to the anode side at an elevated pressure to purge the anode of the ion transport membrane. Flow of the anode side gas has to be countercurrent or cross-countercurrent to that of the cathode side stream. As a result the partial pressure of oxygen at the anode is reduced and a mixture of oxygen and carbon dioxide exits the permeate side. The oxygen contained in the mixture is reacted with fuel in a downstream combustor to form pressurized combustion products consisting primarily of carbon dioxide and steam. Optionally the combustor may be partly or totally integrated with the ion transport membrane by adding fuel to the gas stream entering the anode side. The advantage of integrating part or all of the combustor with the ion transport membrane is increased driving force for oxygen transport. The disadvantage of complete integration is that the peak temperature of the combustion products leaving the membrane unit is limited by the maximum operating temperature of the membrane rather than the turbine which is higher. The gas mixture exiting the combustor at elevated pressure and temperature is subsequently expanded in a gas turbine to generate electric power and exits at a lower pressure and lower temperature. Carbon dioxide is separated from the low pressure, lower temperature, combustion product gas after some recuperative recovery of contained heat. A first retentate portion is conducted from the first oxygen selective ion transport membrane to a cathode side of a second oxygen selective ion transport membrane that is operating at conditions effective to transport a second permeate oxygen portion to an opposing anode side. Nitrogen is recovered from a second retentate portion.
In preferred embodiments of this second aspect, the anode side of the second oxygen selective ion transport membrane may be purged with a mixture of steam and a carbon containing fuel gas. The carbon containing compound may be a hydrocarbon delivered at a mass flow rate effective for a stoichiometric surplus of oxygen on combustion with the second permeate oxygen portion. The second retentate portion may be substantially free of oxygen.
In a third aspect, the invention is directed to a process for the cogeneration of carbon dioxide, oxygen, nitrogen and electrical power. A pressurized oxygen-containing first process gas is flowed along a cathode side of a first oxygen selective ion transport membrane that is at operating conditions effective to transport a first permeate oxygen portion from the cathode side to an opposing anode side. A first retentate portion is flowed along a second cathode side of a second oxygen selective ion transport membrane that is at operating conditions effective to transport a second permeate oxygen portion from the second cathode side to an opposing second anode side. Substantially oxygen-free nitrogen remains in a second retentate portion after the second permeate oxygen portion has been removed. Fuel is introduced into the second permeate passage to react with the second permeate oxygen portion to form first combustion products. The heat of reaction generated by the reaction of second oxygen permeate portion and said fuel is removed primarily by heat transfer to the oxygen containing feed stream flowing through a heat exchange passage in that is integral with said second oxygen transport membrane reactor. Design and flow conditions are controlled to maintain said second oxygen selective ion transport membrane within its appropriate operating temperature range from 800 to 1100xc2x0 C. The first combustion products are flowed along the first anode side and combined with the first permeate oxygen portion to form a second mixture. Heat is removed from the second mixture by transfer of available heat to a pressurized water source thereby forming a pressurized, high temperature, steam and a reduced temperature second mixture. The steam is expanded in a steam turbine to generate electric power. A carbon dioxide-oxygen mixture is separated from the reduced temperature second mixture by condensing out water.
In preferred embodiments of this third aspect, carbon dioxide may be separated from the carbon dioxide-oxygen mixture in a downstream separation using polymeric membranes, pressure or temperature swing adsorption, or partial condensation processes. The steam turbine may be a two-stage steam turbine. The steam turbine may have a first stage exhaust pressure selected to be above to near atmospheric pressure and a second stage exhaust pressure may be at a vacuum pressure of 1 to 5 psia. A portion of the first stage exhaust may be heated and used as additional purge gas for at least one of the membranes.